Selective catalytic reduction (SCR) catalysts may be utilized in the exhaust systems of diesel engines to reduce NOx emissions. A reductant, such as urea, may be injected into the exhaust system upstream of the SCR catalyst, and together, the reductant and the SCR catalyst may chemically reduce NOx molecules to nitrogen and water, thereby limiting NOx emissions. However, if a component of the NOx emission control system, such as the SCR catalyst, becomes degraded, NOx emissions may increase. NOx sensors, configured to measure NOx levels in the exhaust system, may therefore be positioned in the exhaust system to detect failures of the NOx emission control system. Specifically, increases in NOx levels that may be indicative of degradation of one or more components of the NOx emission control system may be detected by the NOx sensors. Thus, the efficiency of the SCR catalyst, and other components of a NOx emission control system may be monitored by one or more NOx sensors positioned in the exhaust system.
NOx sensors may also become degraded, and estimations of the NOx levels may become less accurate. As a result, NOx slippage in the exhaust system may not be detected. In order to monitor the accuracy of a NOx sensor, the NOx sensor may run self-diagnostic (SD) tests after an engine key-off event where an engine is turned off. One such attempt to detect NOx sensor degradation is described in US Patent Application 2014/0144126 to Kowalkowski et al. The disclosure attempts to detect NOx sensor degradation by running one or more SD tests during an engine after-run state.
However, the inventors of the present application have recognized a problem with the above solution. In particular, the disclosed attempt does not address a significant contributor to the NOx levels estimated by the NOx sensor. As one example, urea injected into the exhaust system during engine operation, and/or urea droplets delivered to the exhaust system after the key-off event during a urea delivery line purging process, may persist in the exhaust system after the engine key-off event. Under sufficiently high exhaust temperatures, urea in the exhaust system may be converted to ammonia. NOx sensors register ammonia as NOx, and therefore NOx levels may be overestimated. Therefore, the accuracy of NOx SD tests may decrease with increasing exhaust temperatures and urea concentrations in the exhaust system.
The inventors herein have devised systems and methods for addressing the issues described above. In one example, the issues described above may be addressed by a method comprising: determining that a nitrogen oxide (NOx) sensor is degraded based on outputs received from the sensor via a CAN bus during a self-diagnostic (SD) test performed after a first completed SD test after a key-off event, only if the outputs are generated under conditions where a temperature at the sensor is less than a threshold, a NOx concentration is less than a threshold, and an oxygen concentration is within a threshold range.
In another representation, the issues described above may be addressed by a method comprising: excluding a first completed self-diagnostic (SD) test result of an NOx sensor after an engine key-off event, excluding test results from a completed SD test if one or more of an exhaust gas temperature is greater than a threshold, an oxygen concentration of the exhaust gas is outside a threshold range, and a NOx concentration of the exhaust gas is higher than a threshold, otherwise not excluding test results from a completed SD test, and determining that the sensor is degraded only if the non-excluded test results are different from a reference value by more than a threshold.
In this way, by excluding SD test results from SD tests where the exhaust gas temperature is greater than a threshold, and/or the NOx concentration is greater than a threshold, the variance in the SD test results may be reduced. Said another way, the accuracy of the SD test results may be improved. As such, the sensitivity for distinguishing a degraded NOx sensor from a NOx sensor that is not degraded may be increased. Therefore, the efficiency of a NOx emission control system in an exhaust system may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.